Simply put, she left because she didn't really want to be there in the first place.
Because she felt humiliated by Renato Bruson's encore, an encore that she considers a setup, conductor Gianluigi Gelmetti repeating the aria way too quickly, refusing to either let the applause die down and go ahead from there or simply to barge ahead with the score, using the orchestra to force the applauding crowd into submission. Instead she thinks Gelmetti allowed Bruson to run away with the show.
And because, really, she can leave just like that, with a doctor's note, the way she so often does.
She left because she can and because there was no way she'd go back up there last night singing opposite Bruson, conducted by Gelmetti (who, in fact, does indeed have a reputation for not suffering tantrums lightly).
She's left important productions in important venues for much less, after all.
She stormed off Ravenna Festival's stage, almost ten years ago, a few minutes into her first Pagliacci rehearsal with Riccardo Muti (she had already clashed with Night Porter director Liliana Cavani, a woman of otherwise Vedic calm) -- because Muti corrected her, and suggested Angela did more work on the score. She felt humiliated, and left. Her husband left the production too, just a few days after her, blaming health problems. Riccardo Muti, who prepared the alternate cast in record time, ended in the emergency room -- via ambulance -- just a few hours before la prima, K.O.'d by a renal colic that for all these years people close to the maestro have blamed on the stress caused by the couple's behavior (note to singers felled by random mystery illnesses: Muti asked for a painkiller shot, rested for a few hours in the hospital, then went on to the Teatro Alighieri where he conducted Pagliacci as per his commitment. He never worked with the Gheorghius again).
Back to Rome's Traviata: her husband -- who, if you listen to the inside talk, in Rome and at la Scala, is the reasonable half of the couple, not to mention the nicer half -- had at least the good sense to cancel without bothering to show up -- well, not exactly: he at least had the good manners to tell Zeffirelli that he'd be available only for two shows, giving Zeffirelli the chance to say no thankx in a relatively painless manner; then of course Zeffirelli found himself stuck with a Gheorghiu who waited until the last moment to let him that she'd be available only for two shows, too.
At that point Zeffirelli and Opera di Roma management had to suck it up: firing her would just be nuts, they had been hyping the show as a star-heavy production, very likely the 84-year-old maestro's last new staging of Traviata -- this is his eighth! -- and you just don't pull that off in an emergency with a no-marquee-name, young cast, however talented they may be (and indeed Myrto is a talented young singer). Zeffirelli can pull off awesome stuff like his still powerful flower-children Romeo and Juliet, but this Traviata's concept was much different, and it was impossible to shift the staging's focus. They needed Angela, even for two nights only. For her name and for the hype (and yes, her husband's tantrum at La Scala in a way helped -- bad publicity, no publicity etc), and to their credit at Opera di Roma they're not exactly crying crocodile tears -- they pretented to play along with the "Oh, too bad, I'm sick!" thing. They're just like matches: you play with the Gheorghius, you get burned.
Same for la Scala: they cast her knowing fully the extent of her self-admiration and her cancellation-prone record (they hired two alternate sopranos, just in case); it'll be her first opera there (she only had one recital, last year, and of course there was no way she'd appear at la Scala during Muti's Musical Directorship) and la Scala will happily gather the hype (as much hype as you can get in July anyway) that comes together with her name (and her husband's).
Will she pull another stunt? Well, we hear that it's unlikely she'll sing opposite Leo Nucci anyway -- what if la Scala's audience asks him for the encore he never gave in a 40-year splendid career? (I mean, they love him so much he already led them in a singalong there, wtf!). It'd be the Bruson/Rome fiasco all over again, only 10x worse. So the talk is, she'll sing -- if indeed she'll be there -- opposite the less dangerous Roberto Frontali. And let us hope dear Lorin Maazel keeps his good humor!
Anyway, long post, I know: she left Rome because she knows she can do Traviata in her sleep, at this point. It's her signature role. And she really thinks she sung better, last Friday, than she actually did. Because all those recitals with their yummy fees are taking their tolls on her voice.
Her husband said she cried when he explained her why they called them "the Bonnie and Clyde of opera" -- she hadn't seen the movie.